Hydrosalpinx on Ultrasound: 'Cogwheel Sign' and Incomplete Septa
Ultrasound Semiotics of Hydrosalpinx
Hydrosalpinx is a fluid-filled tubular structure in the adnexal region. According to the literature, three characteristic ultrasound findings are described (Da Li et al., 2025):
| Finding | Plane | Morphological Substrate |
|---|---|---|
| Waist Sign | Sagittal section of the tubal segment | Tubular cystic structure with indentations along the walls, resembling a 'waist'; results from the diameter difference between the wider ampulla and the narrower isthmus of the fallopian tube (Fig. 7.100) |
| Cogwheel Sign | Transverse section | Small bulging echoes (<3 mm) along the inner wall of the tubular cystic structure, resembling a cogwheel |
| Beads-on-a-String | Transverse section | Tiny protrusions into the fluid-filled tube; correspond to fibrously flattened endosalpingeal folds (IOTA, 2026) |
Incomplete Septa
Hydrosalpinx may present as an elongated fluid-filled structure with incomplete septa (Holland, 2026). In the IOTA consensus (2026), a distinction is made between complete septum—as seen in multilocular cysts suspicious for mucinous cystadenoma—and incomplete septa. This differentiation is essential for differential diagnosis.
Cogwheel Sign and Beads-on-a-String
The 'cogwheel' and 'beads-on-a-string' patterns are also encountered in acute or chronic salpingitis, which may present with thickened tubal walls and internal echoes (Holland, 2026). In chronic hydrosalpinx, 'beads-on-a-string' corresponds to fibrously flattened endosalpingeal folds; on color/power Doppler imaging, tiny protrusions into the lumen are visualized (IOTA, 2026).
Differential Diagnosis
Differential diagnosis is important, as the sonographic presentation of tubal cancer overlaps with more common benign pathology (Holland, 2026). The differential includes: hematosalpinx, ectopic pregnancy, tubo-ovarian abscess, and paraovarian masses. Tubal wall thickness, internal echoes, and the character of septa help distinguish these conditions.
Frequently asked questions
What is the size of bulges along the wall in the cogwheel sign?
Small bulging echoes <3 mm along the inner wall of the tubular cystic structure on transverse section (Da Li et al., 2025).
What is the morphological substrate of 'beads-on-a-string'?
Tiny protrusions into the fluid-filled tube correspond to fibrously flattened endosalpingeal folds of the endosalpinx (IOTA, 2026).
How does the waist sign differ from the cogwheel sign?
The waist sign is seen on sagittal section and is caused by the diameter difference between the ampulla and isthmus; the cogwheel sign is seen on transverse section as bulges <3 mm along the wall (Da Li et al., 2025).
How do septa aid in differential diagnosis?
Hydrosalpinx presents as an elongated fluid-filled structure with incomplete septa; complete septa are characteristic of multilocular cysts suspicious for mucinous cystadenoma (Holland, 2026; IOTA, 2026).
What should hydrosalpinx be differentiated from?
Tubal cancer, salpingitis, hematosalpinx, ectopic pregnancy, tubo-ovarian abscess, and paraovarian masses (Holland, 2026).